A simple transformer-based resonator architecture for low phase noise LC oscillators

Abstract

This thesis investigates the use of a simple transformer-coupled resonator to increase the loaded Q of a LC resonant tank. The windings of the integrated transformer replace the simple inductors as the inductive elements of the resonator. The resonator topology considered in this project is a simpler alternative to another proposed by Straayer et al [5] because it just requires a single varactor. A prime objective of this project is to prove that a transformer-coupled resonator which is simpler than that proposed by Straayer in [5] produces the same reduction in phase noise. The use of this type of resonator topology is a valuable technique which can be employed by RF engineers to reduce the phase noise generated by oscillators in high speed RF systems. Such techniques which increase the loaded Q of the resonator are very useful in practice because of the inverse squared relationship between resonator Q and the phase noise in the output signals of LC oscillators. The important aspect of this technique is that magnetic coupling between the windings of an integrated transformer increases their effective inductance while leaving their series resistance relatively unchanged. As a result, the Q of these inductive elements is increased and the phase noise generated by the oscillator is reduced. SpectreRF simulations of an LC oscillator with a center frequency of 5GHz were used to verify the performance of the proposed transformer-coupled resonator.